Dynamo-electric machine

ABSTRACT

Conductor bars  7   a  and end rings  7   b   , 7   c  of a rotor  5  of a rotary electric machine are made of an aluminum material. The end rings  7   b   , 7   c  are electrically and mechanically joined to the opposite ends of the conductor bars  7   a  with friction stir welding.

TECHNICAL FIELD

[0001] The present invention relates to a rotary electric machine, andmore particularly to a rotary electric machine including a rotorsuitable for use in an induction machine.

BACKGROUND ART

[0002] As disclosed in JP,A 61-251440, for example, a rotor of aconventional induction electric motor having a relatively small capacitycomprises a rotor core, conductor bars, and end rings. The rotor core isformed by stacking a plurality of silicon steel sheets one aboveanother, and a plurality of conductor bars are arranged in holes formedin the rotor core. The end rings are fixed to opposite ends of theconductor bars. The conductor bars and the end rings are made ofaluminum or an aluminum alloy. The conductor bars and the end rings areintegrally molded by die casting with high mass production.

DISCLOSURE OF THE INVENTION

[0003] However, because conventional die casting is performed ashigh-pressure casting, there has been a problem that porosity defectsoccur in the conductor bars and the end rings. When a current flowsthrough the conductor bars and the end rings of the rotor, motor torqueis generated with interaction of the current and magnetic flux. Ifporosities are present in a part of the conductor bars and the endrings, it is difficult to improve characteristics of the rotary electricmachine any more because of a reduction of torque caused upon impedimentof the current flow and abnormal overheating in a porosity portion.Also, the presence of the porosity portion causes an unbalance duringthe rotation of the rotor, and hence an unbalance correcting step isessential.

[0004] For the purpose of preventing the occurrence of porosities duringthe die casting, various improvements have proposed in points, forexample, carrying out the die casting in a vacuum, modifying the shapeof a casting gate, preheating dies used in the die casting, andmodifying the surface shapes of dies so that air will not be entrappedduring the die casting. However, a satisfactorily effective solution isnot yet found at present.

[0005] It is an object of the present invention to provide a rotaryelectric machine including a rotor free from a porosity failure.

[0006] To achieve the above object, according to the present invention,in a rotary electric machine comprising a stator and a rotor arranged inan opposed relation to a stator, the rotor comprising a core, aplurality of conductor bars inserted respectively through a plurality ofholes axially formed in the core, and end rings attached to oppositeends of the conductor bars, the conductor bars and the end rings aremade of an aluminum material, and the end rings are electrically andmechanically joined to the opposite ends of the conductor bars withfriction stir welding.

[0007] With such a construction, a rotary electric machine including arotor free from a porosity failure can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a front view, sectioned in an upper half, showing theoverall construction of an induction electric motor according to a firstembodiment of the present invention.

[0009]FIG. 2 is a sectional view showing the construction of a rotorused in the induction electric motor according to the first embodimentof the present invention.

[0010]FIG. 3 is an exploded perspective view for explaining a process ofmanufacturing the rotor used in the induction electric motor accordingto the first embodiment of the present invention.

[0011]FIG. 4 is a perspective view showing a manner of joining the rotorused in the induction electric motor according to the first embodimentof the present invention.

[0012]FIG. 5 is a sectional view showing a step of joining the rotorused in the induction electric motor according to the first embodimentof the present invention.

[0013]FIG. 6 is a plan view showing a state after joining the rotor usedin the induction electric motor according to the first embodiment of thepresent invention.

[0014]FIG. 7 is a sectional view showing a step of joining a rotor usedin an induction electric motor according to a second embodiment of thepresent invention.

[0015]FIG. 8 is a plan view showing a state after joining the rotor usedin the induction electric motor according to the second embodiment ofthe present invention.

[0016]FIG. 9 is a sectional view showing a step of joining a rotor usedin an induction electric motor according to a third embodiment of thepresent invention.

[0017]FIG. 10 is a sectional view showing a step of joining a rotor usedin an induction electric motor according to a fourth embodiment of thepresent invention.

[0018]FIG. 11 is a perspective view showing the construction of a rotorused in an induction electric motor according to a fifth embodiment ofthe present invention.

[0019]FIG. 12 is a sectional view showing the construction of a rotorused in an induction electric motor according to a sixth embodiment ofthe present invention.

[0020]FIG. 13 is a graph for explaining the tensile strength of analuminum alloy.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] The construction of an induction electric motor, i.e., a rotaryelectric machine, according to a first embodiment of the presentinvention will be described below with reference to FIGS. 1 to 6.

[0022] First of all, a description is made of the overall constructionof the induction electric motor according to this embodiment withreference to FIG. 1.

[0023]FIG. 1 is a front view, sectioned in an upper half, showing theoverall construction of the induction electric motor according to thefirst embodiment of the present invention.

[0024] A housing 1 is formed into a substantially cylindrical shape bycasting an iron-base material, such as cast iron, and constitutes acasing of an electric motor. A plurality of heat radiating fins la areprovided on an outer periphery of the housing 1 to extend in the axialdirection and are integrally formed with the housing 1 in a radialpattern. End brackets 2A, 2B are attached to openings at opposite end ofthe housing 1 with spigot joints. A stator 3 comprises stator core 3 aand a stator coil 3 b. The stator 3 is fitted to an inner peripheryportion of the housing 1 and is fixedly held in place. The stator core 3a is formed by stacking a plurality of silicon steel sheets one aboveanother. The stator coil 3 b is wound in a plurality of slots formed inan inner peripheral portion of the stator core 3 a.

[0025] A rotor 5 comprises a stacked core 5 a, conductor bars 7 a, andend rings 7 b, 7 c. The detailed construction of the rotor 5 will bedescribed later with reference to FIG. 2. The rotor 5 is attached to anouter peripheral portion of a rotary shaft 6 in an opposed relation tothe stator 2.

[0026] Opposite ends of the rotary shaft 6 are rotatably held by the endbrackets 2A, 2B through bearings 4A, 4B. Also, one end (on the rightside as viewed in FIG. 1) of the rotary shaft 6 penetrates through theend bracket 2B and is projected to the exterior so as to serve as anoutput shaft. The other end (on the left side as viewed in FIG. 1) ofthe rotary shaft 6 penetrates through the end bracket 2A and is providedwith an external cooling fan (outer fan) 9.

[0027] The end cover 10 covers the outer fan 9. An opening 10 a fortaking in open air is formed in one lateral surface of the end cover 10.Further, the other end of the end cover 10 on the side opposed to theopening 10 a is formed into an open cylindrical shape so that a radialgap 10 b is defined between both outer peripheral portions of the endbracket 2A and of the housing 1 when the end cover 10 is assembled tothe end bracket 2A.

[0028] In the electric motor, when the rotary shaft 6 is driven, theouter fan 9 is rotated and open air is sucked through the opening 10 ain the end cover 10 as indicated by an arrow A. The sucked air passesthrough the gap 10 b and is blown out to the exterior from the other endof the end cover 10. The blown-out air flows along respective surfacesof the end bracket 2A, the heat radiating fins 1 a on the housing 1, andthe end bracket 2B, whereby the cooling action is realized.

[0029] The construction of the rotor of the induction electric motoraccording to this embodiment will be described below with reference toFIG. 2.

[0030]FIG. 2 is a sectional view showing the construction of the rotorused in the induction electric motor according to the first embodimentof the present invention.

[0031] The rotor 5 comprises the stacked core 5 a, the conductor bars 7a, and the end rings 7 b, 7 c. The stacked core 5 a is formed bystacking a predetermined number of thin electromagnetic steel sheets oneabove another. The stacked core 5 a has a plurality of holes 7 d forcage-shaped windings, which are each formed so as to extend in the axialdirection. The plurality of conductor bars 7 a are inserted through theplurality of holes 7 d in a one-to-one relation. The end rings 7 b, 7 care fixed to opposite ends of the plurality of conductor bars 7 a. Theconductor bars 7 a and the end rings 7 b, 7 c constitute a cage-shapedwinding unit. The conductor bars 7 a and the end rings 7 b, 7 c are eachmade of aluminum.

[0032] A method of manufacturing the rotor of the induction electricmotor according to this embodiment will be described below withreference to FIGS. 3 to 6.

[0033] A description is now made of an overall manufacturing process ofthe induction electric motor according to this embodiment with referenceto FIG. 3.

[0034]FIG. 3 is an exploded perspective view for explaining the processof manufacturing the rotor used in the induction electric motoraccording to the first embodiment of the present invention.

[0035] The stacked core 5 a has a plurality of holes 7 d for cage-shapedwindings, which are each formed so as to extend in the radial direction.The plurality of conductor bars 7 a are inserted through the pluralityof holes 7 d in a one-to-one relation. The conductor bars 7 a are each amember formed as an aluminum-made rod. The end ring 7 c is fixed to endsof the plurality of conductor bars 7 a on one side. The end ring 7 c isa member formed as an aluminum-made disk. Also, though not shown in FIG.3, the end ring 7 b is fixed to ends of the plurality of conductor bars7 a on the other side.

[0036] In this embodiment, the rotor 5 is particularly featured in thatthe end rings 7 b, 7 c are integrally joined to the opposite ends of theconductor bars 7 a for the cage-shaped windings with friction stirwelding.

[0037] A manner of joining the conductor bars and the end rings of therotor of the induction electric motor according to this embodiment withfriction stir welding will be described below with reference to FIGS. 4to 6.

[0038]FIG. 4 is a perspective view showing the manner of joining therotor used in the induction electric motor according to the firstembodiment of the present invention, FIG. 5 is a sectional view showinga step of joining the rotor used in the induction electric motoraccording to the first embodiment of the present invention, and FIG. 6is a plan view showing a state after joining the rotor used in theinduction electric motor according to the first embodiment of thepresent invention. Note that the same characters as those in FIGS. 1 to3 denote the same parts.

[0039] As shown in FIG. 4, the end ring 7 c is positioned at an end ofthe stacked core 5 a of the rotor. The conductor bars 7 a are insertedrespectively through a plurality of holes 7 e formed in the end ring 7c. End surfaces of the conductor bars 7 a are substantially flush orcoplanar with an end surface of the end ring 7 c. The end ring 7 c andthe conductor bars 7 a are joined to each other by pressing a frictionstir welding tool 11 against each joint portion.

[0040] The welding tool 11 is a rod-shaped rotating tool and its tip ismade of a material essentially harder than the material of the conductorbars and the end rings, which are each made of aluminum. The material ofthe welding tool 11 is, e.g., alloy tool steel (used for hot dies). Thewelding tool 11 is pressed against each of joint portions between theconductor bars 7 a and the end ring 7 c, and is moved along thecircumference of each of the conductor bars 7 a while rotating about itsown axis. When the welding tool 11 is pressed against the joint portion,frictional heat is generated. With the frictional heat generated, theconductor bar 7 a and the end ring 7 c are caused to plastically flowfor stir welding, whereby both the members are integrally joined to eachother.

[0041] A description is now made of a state during joining of the rotorwith reference to FIG. 5. The conductor bar 7 a has a diameter R1 of,e.g., 20 mm. The end ring 7 c has a thickness H1 of, e.g., 20 mm. Thetip portion of the welding tool 11 has a diameter R2 of, e.g., 5 mm.

[0042] The tip of the welding tool 11 is pressed against the jointportion between the conductor bar 7 a and the end ring 7 c under apressing force F. The pressing force F applied at that time is about 10tons. The welding tool 11 is rotated at the number of rotations of,e.g., 1000 r/min. Upon the rotating welding tool 11 being pressedagainst the joint portion between the conductor bar 7 a and the end ring7 c, frictional heat is generated, causing the conductor bar 7 a and theend ring 7 c to plastically flow for stir welding, whereby a weldportion 7 f is formed. The welding tool 11 is moved along thecircumference of the conductor bar 7 a while rotating about its ownaxis. The moving speed of the welding tool 11 on that occasion is setto, e.g., 700 mm/min. The number of rotations and the moving speed ofthe welding tool 11 differ depending on the kinds, thicknesses, etc., ofmaterials to be joined together. The number of rotations is set withinthe range of, e.g., about 500 to 2000 r/min, and the moving speed is setwithin the range of, e.g., about 200 to 1200 mm/min.

[0043]FIG. 6 shows a state of the weld portion after the welding. In theweld portion 7 f, recessed marks are formed because of the pressingforce F applied from the rotating tool 11 and the generated frictionalheat.

[0044] By employing the friction stir welding method, the conductor bar7 a and the end ring 7 c are joined to each other in solid phase whilethe temperature of the weld portion is kept not higher than the meltingpoint (660° C.) of aluminum. Therefore, the weld portion contains fewerstrains and is free from defects such as bubbles and cracks. On theother hand, the strength of the weld portion is comparable to or higherthan that obtainable with MIG welding, and neither sputters nor fumesare generated. Further, the friction stir welding method does notrequire skills, and a welding apparatus is relatively inexpensive. Sincethe conductor bar and the end ring are not formed by aluminum diecasting that has been used in the prior art, structural defects, such asporosities, can be avoided. Accordingly, characteristics of the rotaryelectric machine can be improved. In addition, because of the absence ofporosities, an unbalance does not occur during the rotation of therotor, and hence an unbalance correcting step is no longer required.

[0045] Further, the aluminum die casting method requires a high energycost because of the necessity of melting aluminum at about 700° C., andalso causes an environmental problem due to combustion for the melting.In contrast, in this embodiment utilizing the friction stir weldingmethod, the energy cost can be cut down and the environmental problem isprevented.

[0046] The manufacturing process may comprise the steps of integrallyjoining one 7 b of the end rings 7 b, 7 c and the conductor bars 7 awith the friction stir welding method, inserting an obtained integralassembly through the holes 7 d for cage-shaped windings formed in therotor core 5 a, and then integrally joining the remaining end ring 7 cto the other ends of the conductor bars 7 a in a similar manner with thefriction stir welding.

[0047] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0048] Next, the construction of an induction electric motor, i.e., arotary electric machine, according to a second embodiment of the presentinvention will be described below with reference to FIGS. 7 to 8. Theoverall construction of the induction electric motor according to thisembodiment is the same as that shown in FIG. 1. The construction of arotor of the induction electric motor according to this embodiment isthe same as that shown in FIG. 2. In this second embodiment, thefriction stir welding is utilized as in the first embodiment shown inFIGS. 1 to 6, but a welding manner differs from that in the firstembodiment.

[0049]FIG. 7 is a sectional view showing a step of joining a rotor usedin the induction electric motor according to the second embodiment ofthe present invention, and FIG. 8 is a plan view showing a state afterjoining the rotor used in the induction electric motor according to thesecond embodiment of the present invention. Note that the samecharacters as those in FIGS. 1 to 6 denote the same parts.

[0050] As shown in FIG. 7, a tip of a welding tool 11′ used in thisembodiment has a diameter R2′ larger than the diameter R1 of theconductor bar 7 a. The tip of the welding tool 11′ is pressed against ajoint portion between the conductor bar 7 a and the end ring 7 c under apressing force F′. The pressing force F′ is proportional to an area ofthe joint portion. Assuming the tip of the welding tool 11′ to have adiameter of 25 mm, therefore, the pressing force of about 250 tons isrequired. The welding tool 11′ is rotated at the number of rotations of,e.g., 1000 r/min. Upon the rotating welding tool 11 being pressedagainst the joint portion between the conductor bar 7 a and the end ring7 c, frictional heat is generated, causing the conductor bar 7 a and theend ring 7 c to plastically flow for stir welding, whereby a weldportion 7 f′ is formed. The welding tool 11′ is moved in thecircumferential direction of the end ring 7 c while rotating about itsown axis.

[0051]FIG. 8 shows a state of the weld portion after the welding. In theweld portion 7 f′, recessed marks are formed because of the pressingforce F applied from the rotating tool 11′ and the generated frictionalheat.

[0052] By employing the friction stir welding method, the conductor bar7 a and the end ring 7 c are joined to each other in solid phase whilethe temperature of the weld portion is kept not higher than the meltingpoint (660° C.) of aluminum. Therefore, the weld portion contains fewerstrains and is free from defects such as porosities. Because of theabsence of porosity defects, it is possible to not only avoid thepossibility that a current flowing through the conductor bars and theend rings of the rotor is impeded and torque is not produced, but alsoprevent a trouble from being caused by abnormal heating in a porosityportion. Further, because of the absence of porosities, an unbalancedoes not occur during the rotation of the rotor, and hence an unbalancecorrecting step is no longer required.

[0053] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0054] Next, the construction of an induction electric motor, i.e., arotary electric machine, according to a third embodiment of the presentinvention will be described below with reference to FIG. 9. The overallconstruction of the induction electric motor according to thisembodiment is the same as that shown in FIG. 1. The construction of arotor of the induction electric motor according to this embodiment isthe same as that shown in FIG. 2. In this third embodiment, the frictionstir welding is utilized as in the first embodiment shown in FIGS. 1 to6, but the conductor bar and the end ring have different constructionsfrom those in the first embodiment.

[0055]FIG. 9 is a sectional view showing a step of joining the rotorused in the induction electric motor according to the third embodimentof the present invention. Note that the same characters as those inFIGS. 1 to 6 denote the same parts.

[0056] In the embodiment shown in FIG. 5 or 7, the conductor bar 7 apenetrates through the end ring 7 c. On the other hand, in thisembodiment, a spigot portion 7 g is provided at a joint portion betweena conductor bar 7 a′ and an end ring 7 c′, and these two members arefitted to each other with a spigot joint. A tip of a welding tool 11″ ispressed against the spigot portion 7 g and is rotated at the same time.With the rotation, frictional heat is generated, causing the conductorbar 7 a′ and the end ring 7 c′ to plastically flow for stir welding. Thewelding tool 11′ is moved along the spigot portion 7 g in the directiontoward an outer periphery of the end ring 7 c′ while rotating about itsown axis.

[0057] By employing the friction stir welding method, the conductor bar7 a′ and the end ring 7 c′ are joined to each other in solid phase whilethe temperature of a weld portion is kept not higher than the meltingpoint (660° C.) of aluminum, and therefore structural defects such asporosities are avoided. Because of the absence of porosity defects, itis possible to not only avoid the possibility that a current flowingthrough the conductor bars and the end rings of the rotor is impeded andtorque is not produced, but also prevent a trouble from being caused byabnormal heating in a porosity portion. Further, because of the absenceof porosities, an unbalance does not occur during the rotation of therotor, and hence an unbalance correcting step is no longer required.

[0058] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0059] Next, the construction of an induction electric motor, i.e., arotary electric machine, according to a fourth embodiment of the presentinvention will be described below with reference to FIG. 10. The overallconstruction of the induction electric motor according to thisembodiment is the same as that shown in FIG. 1. The construction of arotor of the induction electric motor according to this embodiment isthe same as that shown in FIG. 2. In this fourth embodiment, thefriction stir welding is utilized as in the first embodiment shown inFIGS. 1 to 6, but the conductor bar has a different shape from that inthe first embodiment.

[0060]FIG. 10 is a sectional view showing a step of joining the rotorused in the induction electric motor according to the fourth embodimentof the present invention. Note that the same characters as those inFIGS. 1 to 6 denote the same parts.

[0061] While the conductor bar 7 a shown in FIG. 6 is circular in crosssection, a conductor bar 7 a″ in this embodiment has a cross-sectionalshape of a tear droplet as shown in FIG. 10. An end ring 7 c″ has holes7 e″ formed therein for penetration of the conductor bars 7 a″ throughthe holes 7 e″. Though not shown, holes each having a shapecorresponding to the shape of the conductor bar 7 a″ are formed in astacked core 5 a″. The conductor bars 7 a″ are inserted respectivelythrough those holes.

[0062] The tip of the rotating welding tool is pressed against a jointportion between the conductor bar 7 a″ and the end ring 7 c″. With thepressing of the rotating welding tool, frictional heat is generated,causing the conductor bar 7 a″ and the end ring 7 c″ to plastically flowfor stir welding. The welding tool is moved along a weld portion betweenthe conductor bar 7 a″ and the end ring 7 c″ while rotating about itsown axis.

[0063] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0064] Next, the construction of an induction electric motor, i.e., arotary electric machine, according to a fifth embodiment of the presentinvention will be described below with reference to FIG. 11. The overallconstruction of the induction electric motor according to thisembodiment is the same as that shown in FIG. 1. A rotor of the inductionelectric motor according to this embodiment is a skewed rotor.

[0065]FIG. 11 is a perspective view showing the construction of a rotorused in the induction electric motor according to the fifth embodimentof the present invention. Note that the same characters as those inFIGS. 1 to 6 denote the same parts.

[0066] In this embodiment, a rotor 5 is formed as described withreference to FIGS. 1 to 6. Then, a skewed rotor 5′ can be obtained byapplying, to the rotor 5, a twisting force in the rotating direction sothat a skew is given to conductor bars. In FIG. 11, the skewed rotor 5′comprises a stacked core 5 a, end rings 7 b, 7 c, and a plurality ofconductor bars 7 a.

[0067] A manner of forming the skewed rotor 5′ may comprise the steps asfollows. When electromagnetic steel sheets are stacked to form the core,the core is formed by stacking those steel sheets such that a skew isgiven to the core in advance. Similarly, the conductor bars are eachformed into such a sectional shape as giving a skew to the conductorbars in advance. Then, the conductor bars are axially inserted throughholes 7 d for cage-shaped windings. Then, the conductor bars and the endrings are integrally joined to each other with the friction stir weldingin a similar manner to that described above.

[0068] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0069] Also, since the skewed rotor can suppress noise and pulsations intorque at the startup in comparison with the rotor having no skew, theperformance of the electric motor can be improved.

[0070] Next, the construction of an induction electric motor, i.e., arotary electric machine, according to a sixth embodiment of the presentinvention will be described below with reference to FIGS. 12 and 13. Theoverall construction of the induction electric motor according to thisembodiment is the same as that shown in FIG. 1.

[0071] A description is first made of the construction of a rotor usedin the induction electric motor according to this embodiment withreference to FIG. 12.

[0072]FIG. 12 is a sectional view showing the construction of the rotorused in the induction electric motor according to the sixth embodimentof the present invention. Note that the same characters, as those inFIGS. 1 to 6 denote the same parts.

[0073] A rotor 5A comprises a stacked core 5 a, conductor bars 7A, andthe end rings 7B, 7C. The stacked core 5 a is formed by stacking apredetermined number of thin electromagnetic steel sheets one aboveanother. The stacked core 5 a has a plurality of holes 7 d forcage-shaped windings, which are each formed so as to extend in the axialdirection. The plurality of conductor bars 7A are inserted through theplurality of holes 7 d in a one-to-one relation. The end rings 7B, 7Care fixed to opposite ends of the plurality of conductor bars 7A. Theconductor bars 7A and the end rings 7B, 7C constitute a cage-shapedwinding unit. The conductor bars 7A are each made of aluminum. However,unlike the embodiments described above, the end rings 7B, 7C are eachmade of an aluminum alloy.

[0074] Here, the relationship between tensile strength and temperatureof an aluminum alloy will be described with reference to FIG. 13.

[0075]FIG. 13 is a graph for explaining the tensile strength of analuminum alloy. In the graph of FIG. 13, the horizontal axis representstemperature T (° C.) and the vertical axis represents tensile strength S(%).

[0076] A sold line A indicates dependency of the tensile strength ofaluminum with purity of 99.7% upon temperature. A sold line B indicatesdependency of the tensile strength of an aluminum alloy upontemperature. In FIG. 13, on an assumption that the tensile strength ofan aluminum alloy at 20° C. is set to 100%, the tensile strength atanother temperature and tensile strength of aluminum are plotted asrelative values.

[0077] The term “aluminum alloy” used herein means a die-cast aluminumalloy called ADC12 and is an Al—Si—Cu based alloy in which Si is in therange of 10.5 to 12%. The tensile strength of the aluminum alloy isabout 300 MN/m² at room temperature and is about twice that of purealuminum. Also, the aluminum alloy has such temperature characteristicsthat the tensile strength is reduced just about 25% at 150 (° C.) andensures a strong property at relatively high temperature. In otherwords, as shown in FIG. 13, the aluminum alloy has greater tensilestrength and a less reduction in strength at relatively high temperaturethan aluminum.

[0078] In the induction electric motor, a current flows through therotor based on electromagnetic induction with magnetic flux generatedfrom the stator. While a rotating force is applied to the rotor underthe action of the magnetic flux and the current, Joule heat is producedwith the current flowing through the rotor and hence the temperature ofthe rotor is increased. It is general that the temperature of the rotoris about 150 (° C.) in ordinary condition of use.

[0079] Recently, the size of induction electric motors has been reducedand the number of rotations of induction electric motors in use has beenincreased in many cases. In the field of grinders, for example, themachining accuracy is improved by increasing the number of rotations ofa main spindle and increasing the circumferential speed of a blade. Thecircumferential speed of electric motors used in the field of grindershas been 150 m/min at maximum in the past, but a circumferential speedof 200 m/min is demanded at present.

[0080] While in a conventional induction electric motor electromagneticsteel sheets, conductor bars and end rings constituting a rotor aregiven with high tension, the conductor bars, etc. are made of a non-ironmetal, such as aluminum. Accordingly, the highly tensile electromagneticsteel sheets have a higher tensile strength of the material itself, andthe allowable maximum number of rotations is decided depending on themechanical strength of the material of the conductor bars including theend rings.

[0081] Calculating the strength of a rotor rotating at a high speed, itis understood that inner peripheral stresses in an inner diameterportion of the stacked core and inner peripheral stresses of the endrings are high. Because, of the components of the rotor, the tensilestrength of the end ring material is smaller than that of the stackedcore, the strength of the rotor is decided depending on the innerperipheral stresses of the end ring.

[0082] For that reason, in this embodiment, the end rings are made of analuminum alloy that has a high tensile strength and a less reduction instrength at high temperatures. Then, hetero-metals, i.e., the conductorbars 7A made of pure aluminum and the end rings 7B, 7C made of thealuminum alloy, are integrally joined to each other with the frictionstir welding described above, whereby the rotor is constructed.

[0083] The specific resistance of the aluminum alloy is 7.3 μΩcm, whilethe specific resistance of the pure aluminum is 3.4 μΩcm. In otherwords, the specific resistance of the aluminum alloy is twice that ofthe pure aluminum. Accordingly, if the aluminum alloy is used as theconductor bar, Joule heat produced by the rotor is increased twice andthe temperature of the rotor rises correspondingly. Also, slippage ofthe rotor is doubled, which may result in a reduction of performance. Inthis embodiment, therefore, pure aluminum is used as the material of theconductor bars 7A.

[0084] On the other hand, if the aluminum alloy is used as the endrings, Joule heat is generated in an increased amount. However, byreducing the inner diameter of the end rings 7B, 7C, as shown in FIG.12, to increase the cross-sectional area of the end rings 7B, 7C ascompared with that of the end rings 7 b, 7 c shown in FIG. 2, theresistance value of each end ring shown in FIG. 12 is made equal to thatof each end ring shown in FIG. 2. Hence, the end rings shown in FIG. 12generates Joule heat comparable to that of the end rings shown in FIG.2. Since the performance of an electric motor depends on not only theresistance value, but also the shape and dimensions of the conductorbars, electric motors compatible in performance cannot be provided onlywhen the resistance value is equal. However, the factor of the end ringsaffecting the performance of an electric motor is only the resistancevalue of the end rings.

[0085] As the aluminum alloy, hydronalium, for example, can be used inaddition to ADC12. Hydronalium is an Al—Mg—base alloy in which Mg is inthe range of 7 to 9%. The tensile strength of hydronlrium is 18 kg/mm²,which is higher than that (9 to 17 kg/mm²) of the pure aluminum.

[0086] With this embodiment, as described above, a superior rotor can beobtained because no porosities are caused and the weld portion is freefrom strains.

[0087] Also, a rotor having a higher strength and being able o rotate athigh speeds can be obtained.

Industrial Applicability

[0088] According to the present invention, a rotary electric machineincluding a rotor free from porosities can be provided.

1. A rotary electric machine comprising a stator and a rotor arranged inan opposed relation to said stator, said rotor comprising a core, aplurality of conductor bars inserted respectively through a plurality ofholes axially formed in said core, and end rings attached to oppositeends of said conductor bars, wherein said conductor bars and said endrings are made of an aluminum material, and said end rings areelectrically and mechanically joined to the opposite ends of saidconductor bars with friction stir welding.
 2. A rotary electric machinecomprising a stator and a rotor arranged in an opposed relation to saidstator, said rotor comprising a core, a plurality of conductor barsinserted respectively through a plurality of holes axially formed insaid core, and end rings attached to opposite ends of said conductorbars, wherein said conductor bars and said end rings are made of analuminum material, and said end rings are electrically and mechanicallyjoined to the opposite ends of said conductor bars with friction stirwelding such that a locus formed by the friction stir welding is drawnin conformity with a shape of said end ring.
 3. A rotary electricmachine comprising a stator and a rotor arranged in an opposed relationto said stator, said rotor comprising a core, a plurality of conductorbars inserted respectively through a plurality of holes axially formedin said core, and end rings attached to opposite ends of said conductorbars, wherein said conductor bars and said end rings are made of analuminum material, and said end rings are electrically and mechanicallyjoined to the opposite ends of said conductor bars with friction stirwelding such that a plurality of loci formed by the friction stirwelding are drawn in conformity with shapes said conductor bars.
 4. Arotary electric machine comprising a stator and a rotor arranged in anopposed relation to said stator, said rotor comprising a core, aplurality of conductor bars inserted respectively through a plurality ofholes axially formed in said core, and end rings attached to oppositeends of said conductor bars, wherein said conductor bars and said endrings are made of an aluminum material, and end surfaces of saidconductor bars and one lateral surface of said end ring are electricallyand mechanically joined to each other with friction stir welding.
 5. Arotary electric machine comprising a stator and a rotor arranged in anopposed relation to said stator, said rotor comprising a core, aplurality of conductor bars inserted respectively through a plurality ofholes axially formed in said core, and end rings attached to oppositeends of said conductor bars, wherein said conductor bars and said endrings are made of an aluminum material, and said conductor bars arefitted to a groove formed in said end ring, and a peripheral surface ofsaid end ring is electrically and mechanically joined to one surfaces ofsaid conductor bars with friction stir welding.
 6. A rotary electricmachine according to any one of claims 1 to 5, wherein an aluminum alloyis used as the aluminum material forming said end rings.
 7. A rotaryelectric machine according to any one of claims 1 to 5, wherein purealuminum alloy is used as the aluminum material forming said conductorbars, and an aluminum alloy is used as the aluminum material formingsaid end rings.
 8. A rotary electric machine according to any one ofclaims 1 to 5, wherein said rotor is a skewed rotor.